1. Field of the Invention
The present invention relates to a substrate bonding method, a bonded product, an ink jet head, and an image forming apparatus. More particularly, the present invention is concerned with a substrate bonding method, a bonded product, an ink jet head, and an image forming apparatus, which are to be used in fabricating functional devices by various micromachining techniques.
2. Description of the Prior Art
According to a substrate bonding method so far adopted generally, an adhesive is used for bonding. However, as to functional devices fabricated using various micromachining techniques, there is a conspicuous tendency to microsizing and, in bonding using an adhesive, an inappropriate thickness of the adhesive and protrusion thereof give rise to a serious problem.
For example, as an ink jet head fabricating method, there has been proposed the use of two substrates bonded together, typical of which is disclosed in Japanese Published Unexamined Patent Application No. Sho 61-230954. However, as ink flowing grooves formed in bonding surfaces become higher in density and smaller in size, the application of an adhesive used in bonding becomes difficult. As a remedial measure there is proposed in Japanese Published Unexamined Patent Application No. Sho 63-34152 a technique in which an adhesive is applied thin and selectively.
Recently, however, there has been an increasing tendency to a smaller dot size with a view to attaining a high image quality in an ink jet head, and the size of a nozzle as an ink jet orifice is also becoming very small to meet the tendency to smaller dots. Such a reduction of the nozzle size is giving rise to the following several problems also in the method proposed in Japanese Published Unexamined Patent Application No. Sho 63-34152.
(1) The groove size in bonded surfaces is becoming extremely small with the recent tendency to a smaller nozzle size, and even a slight protrusion of an adhesive results in the nozzle and ink flowing grooves being filled up with the adhesive. FIG. 9 shows ink jet head nozzle portions. The nozzle shown in FIG. 9A is a conventional 400 DPI equivalent nozzle having an ink droplet generating width of about 20 xcexcm, and with this size, there is no fear of the nozzle being filled up with an adhesive 70 even by protrusion of the adhesive. But in case of such a reduced-size nozzle of about 5 xcexcm in width as in FIG. 9B, which is equivalent to 1600 DPI and which generates ink droplets, the entire nozzle opening is filled up with protruded adhesive 70 and cannot eject ink.
(2) If the adhesive coating is made thin for preventing the foregoing protrusion of the adhesive into the ink flowing paths, there will occur defective sealing between ink flowing paths due to adhesive deficiency in a poor adhesion area which is attributable to the presence of fine convexes and concaves in bonding substrate surfaces or substrate warping, with consequent occurrence of an inconvenience such that a pressure developed for the ejection of ink in a certain ink flowing path escapes to an ink flowing path adjacent thereto. Therefore, for the purpose of improving the flatness of bonding substrate, surfaces there is adopted, for example, a planarizing process (Japanese Unexamined Patent Application No. Hei 11-245426) using CMP (Chemical Mechanical Polish). In bonding substrates of a large area, however, due to warping of wafers or the presence of particles, a substrate-to-substrate gap (bonding gap) of about 1.0 xcexcm or less occurs in the foregoing defective adhesion area.
(3) As a result of having used the ink jet nozzle for a long period of time, a thin adhesive film which has been transferred to a protruded portion of the adhesive or to an unbonded portion comes off and may cause clogging of the nozzle. FIG. 10A shows an ink flowing path structure in the ink jet head and FIG. 10B is a sectional view taken on line Xxe2x80x94X in FIG. 10A. In the case where a thin adhesive film is transferred to a convex portion formed on an ink flowing path substrate 40 and the substrate 40 and a device substrate 26 are bonded together, there occurs a state in which the adhesive is transferred but bonding is not made at rear wall portions of individual flow paths 44 corresponding to bypaths.
In an effort to solve the abovementioned problems, various techniques have been proposed which do not use any adhesive, such as eutectic bonding and room temperature interface bonding. In eutectic bonding, however, it is necessary to form a thin Au film, resulting in an increase of the manufacturing cost. In room temperature interface bonding, it is necessary to perform a special interface treatment using an ultra-high vacuum or an ion beam, thus requiring the use of special equipment.
Further, in anodic bonding, it is necessary to apply a voltage as high as several hundred volts, with consequent danger of transistors, etc. being destroyed in case of a substrate having an electronic circuit.
As a bonding method which is carried out through a resin layer, there is known a thermocompression bonding, but it is necessary to apply a high pressure (4.9xc3x97105 to 9.8xc3x97105 Pa), thus requiring the use of special equipment. In addition, there arises a problem in point of reliability of a circuit-mounted board because of application of a high pressure. In forming fine patterns, there arises the same problem as in the use of an adhesive in point of thickness and protrusion of a resin layer.
The present invention has been accomplished for solving the abovementioned problems and provides a substrate bonding method, a bonded product obtained by using the substrate bonding method, an ink jet head, and an image forming apparatus, capable of implementing a satisfactory bonded state at low cost.
For solving the foregoing problems, in the substrate bonding method according to the present invention, a resin material is sandwiched between plural substrates and voltage is applied across the plural substrates to bond the substrate together. In the present invention, a plus side of a resin material which has been dipole-polarized by the application of voltage across plural substrates and a substrate on a minus electrode side attract each other by an electrostatic attraction, while a minus side of the resin material and a substrate on a plus electrode side attract each other by an electrostatic attraction, with the result that the resin material gets in fine concaves and convexes on the substrate interface, thus creating an anchor effect, whereby the substrates are bonded together through the resin material. Further, it is presumed that minus ions migrate from the resin material to the substrate side and undergoes a chemical reaction with the material of the substrate, with consequent chemical bonding reinforcing the substrate bonding. Thus, the substrate bonding does not require the use of any adhesive, nor is it necessary to use any equipment for implementing high temperature and high pressure conditions. Consequently, a satisfactory bonding free of adhesive protrusion can be attained at low cost.
It is preferable that the applied voltage be 350V or lower and be 50V or so. With such a level of voltage, the resin material is shifted moderately and a satisfactory bonding is obtained; in addition, since such a high voltage of several hundred volts as in the anodic bonding is not applied, it is not necessary to use any special equipment and bonding can be done at low cost.
By applying pressure to the plural substrates at the time of the voltage application there can be attained a better bonded state.
It is preferable that the plural substrates be held at a temperature of 200xc2x0 C. or higher at the time of the voltage application.
It is preferable that a maximum temperature of the plural substrates at the time of the voltage application be a heat-resisting temperature of the resin material or lower and a glass transition point temperature of the resin material or higher. If the substrate temperature is the glass transition point temperature of the resin material or higher, it becomes easier for the resin material to move, affording a good bonded state; in addition, there is little residual stress at completed bonding.
By maintaining the resin material before the bonding in an uncured state and by allowing it to cure completely simultaneously with or after the bonding, there can be obtained a good bonded state without the need of planarizing bonding surfaces.
At least one of the plural substrates is formed by silicon.
The substrate bonding method according to the present invention can adopt the procedure of raising the temperature of the plural substrates to a first predetermined temperature, applying a predetermined pressure to the plural substrates, applying a predetermined voltage across the plural substrates, starting to reduce the temperature of the plural substrates after the value of an electric current flowing across the plural substrates has reached a level of a predetermined current value or lower or upon the lapse of a predetermined time after the voltage application, and stopping the application of the voltage and the application of the pressure after the temperature of the plural substrates has dropped to a second predetermined temperature or lower.
The application of the pressure across the plural substrates may be performed in at least two stages of pressure levels during the rise in temperature of the plural substrates. According to this bonding method, since the pressure applied to the substrates is increased against shrinkage of the resin material with an increase in temperature, a better bonded state can be obtained while maintaining a certain gap on the bonded surfaces.
By forming a metallic pattern on at least one of the plural substrates with bonding surfaces of the substrates therebetween, the potential on the substrate upon the application of voltage becomes almost uniform, thus affording a uniform and satisfactory bonded state.
If the bonding surfaces of at least one of the plural substrates and the resin material are subjected to cleaning and activation before the bonding, it will be possible to obtain the foregoing anchor effect more efficiently and also possible to effect a chemical reaction in a satisfactory manner, so that a good bonded state can be attained.
The bonded product according to the present invention is obtained by the above substrate bonding method.
The ink jet head according to the present invention is constituted by the above bonded product.
In the ink jet head according to the present invention, on one of a pair of substrates which constitute the bonded product, there is formed a pattern including fine ink flowing grooves while on the other substrate is formed a pattern including plural circuits.
In the present invention it is also possible to constitute an image forming apparatus provided with the above ink jet head.